The aims of this study are to (1) determine whether or not propranolol's inhibition of isoproterenol-stimulated response is purely competitive or mixed; (2) determine whether propranolol's inhibition of isoproterenol- stimulated response is time and/or concentration dependent; and (3) examine the mechanism and location (receptor, post-receptor) of propranolol's inhibition of isoproterenol-stimulated response.